Myasthenia gravis (MG) and its animal models Experimental Autoimmune Myasthenia Gravis (EAMG), are characterized by an ease of fatigability and relative weakness of voluntary muscles. A common feature associated with MG is T lymphocyte-dependent serum antibody responses against a self- antigen expressed by voluntary muscles, the acetylcholine receptor (AChR); the binding of anti-AChR autoantibodies to AChR at the neuromuscular junction results in the observed impairment of neuromuscular transmission. Various modalities of therapy are used to treat MG, most often immunosuppressive chemotherapy. However, the innate lack of selectivity of the drugs employed limits their usefulness. The goal of the proposed project is to develop a more selective immunosuppressive agent. Thus, as a key feature of the anti-AChR antibody response in experimental MG is that it is a helper T lymphocyte-dependent response, selective killing of the AChR-reactive T cells should result in the inhibition of the subsequent antibody response to AChR with limited effects on immune responses to unrelated antigens. Therefore, the goal of the proposed studies is to construct agents capable of selectively deleting AChR-immune helper T cells. Binding of a monoclonal antibody that recognizes a region of the beta chain of the antigen receptor expressed by only about 5% of all T cells in C57BL/6 mice and the majority of AChR-reactive helper T cells (i.e., Vbeta6-associated T cells) could supply the desired selectivity. Cell-specific antibodies covalently linked to the A subunit of the toxin ricin (Ricinus communis agglutinin II) has received much attention in connection with targeted therapeutic strategies. Used as the binding moiety in an "immunotoxin", anti-Vbeta6 antibodies should promote the killing of AChR-reactive T cells and significantly limit the breadth of cytotoxicity and immunosuppression compared to chemotherapeutic drugs. The general scheme and goals of the proposed experimentation would be as follows (the project is a straight-forward extension of ongoing work by the investigators and involves the novel application of existing technology): (i) Prepare Vbeta6-specific immunotoxins (VIT6) and test in vitro for cytotoxicity against monoclonal and polyclonal AChR-reactive T cells; (ii) Test Vbeta6-specific immunotoxins in vitro for the ability to decrease inducible anti-AChR antibody production reflecting lost helper T cell activity; (iii) Test Vbeta6-specific immunotoxins in vivo for the ability to protect mice prior to disease induction; and (iv) Test Vbeta6-specific immunotoxins in vivo for the ability to interfere with ongoing anti-AChR immune responses and progressive disease.